A Multi-Leaf Collimator (MLC) is used in external beam radiation therapy in order to collimate the radiation beam to a chosen cross-sectional shape. The aim is to allow accurate delivery of the radiation to the tumour volume, and this is achieved by collimating the beam with an array of narrow elongate tungsten leaves, arranged side-by side and individually motorised in order to allow them to be moved longitudinally. This allows the collimator to define a desired shape.
There are limitations on the longitudinal length of the leaves, dependent on their thickness. Generally, for a given leaf thickness (which does of course define the collimator resolution), there is a maximum feasible length of leaf. Beyond this length, difficulties arise in driving the leaf reliably. This therefore limits the maximum possible collimator aperture, and hence the size of the volume that can be treated. A compromise must therefore be reached between resolution and aperture.
To allow for a greater aperture without compromising resolution, some MLCs mount the leaves on a secondary motion axis, usually referred to as a “carriage”. Thus, the leaves are supported on and movable relative to the carriage, and the carriage is mounted on and moveable longitudinally relative to a substrate. This allows collimated shapes to be made in a larger field size than that of a leaf alone. The movements are operated as two distinct axes, operating either on their own or sequentially.